1. Field of the Invention
The present invention is directed to torque transferring low carbon steel shafts, for example, drive shafts for motor vehicles, and to a process for their preparation by carboaustempering with grain refinement.
2. Background Art
It has been known for many centuries that the physical characteristics of steel are strongly dependent upon its thermal and mechanical history. Frequently, steel parts are provided in their final shape or near net shape, and thus mechanical operations such as forging, rolling, etc., cannot be further used to alter physical properties. However, thermal treatments are still available for use with such parts.
It is modernly understood that the microstructure of metals and metal alloys can be quite complex. Even in plain-carbon steel and other low alloy steels where carbon is the principal non-ferrous ingredient, a variety of phases are known to exist, for example, the cubic face centered structure of austenite and the body centered tetragonal structure of martensite, as well as ±-ferrite, cementite, ledeburite, pearlite, and bainite. Transformation of one phase to another may take place within certain temperature ranges, and often, the degree to which a transformation takes place will be markedly affected by quench rates. For example, if the quench rate is high, the steel may be “frozen” in a morphology which is incapable of being formed in samples which are slow cooled. For these reasons, there are a myriad of possible heat treatment processes, each of which generates its own combination of physical characteristics, such as hardness, tensile strength, elongation, ductility, etc. In addition, samples of steel having had different thermal histories can exhibit markedly different fatigue resistance.
In addition to heat treatments which can, depending upon part size and geometry, affect the entire structure, there are heat treatments which affect mainly the outside of the structure, for example, carburizing which may be used to surface harden parts (“case hardening”) to achieve a more wear resistant and harder exterior combined with a more ductile interior.
Power transmission shafts must be strong and fatigue resistant. The stresses imparted to such shafts is rarely constant, and even in “constant speed” devices, the loads are generally cyclical. In the vehicle sector, loads can vary widely. Moreover, power transmission shafts often have features such as splines, holes for lubrication, etc., which often lower fatigue resistance at these points. The strength and resistance to fatigue for such parts can be increased by choosing a stronger alloy steel, but this solution involves considerable extra expense. A larger section shaft can also be used, but this solution uses more space, often restricted by design, and also involves a considerable weight penalty.
Typically, such shafts are induction hardened, as illustrated in U.S. Pat. Nos. 6,319,337 and 6,390,924, which employ induction hardened low alloy steel. However, with ever increasing loads coupled with the desire to keep weight as low as possible, induction hardening has not proven satisfactory in providing the fatigue resistance desired.
It would be desirable to provide steel power transmission shafts which offer improved fatigue resistance without employing highly alloyed steels, and without increasing the size and weight of the shaft.